FIG. 1 is a block diagram of a prior-art holographic radar shown in the paper "Digital Multiple Beamforming Techniques for Radar" by Abraham E. Ruvin and Leonard Weinberg in EASCON-78 Record, a publication of the Institute of Electrical and Electronics Engineers (IEEE). The radar shown in FIG. 1 comprises N antenna elements 1 forming an antenna 2, RF amplifiers 3 connected to the antenna elements to amplify the radio-frequency signal received by the antenna element, mixers 4 which convert the received RF signal to an intermediate-frequency signal, IF amplifiers 5 which amplify the intermediate-frequency signal output from the mixers 5, phase detectors 6 which convert the output of the IF amplifiers to a baseband complex video signal while preserving the phase of the intermediate-frequency signal, low-pass filters 7 connected to the I (in-phase) channel output and Q (quadrature) channel output of the phase detectors 6, A/D converters 8 connected to the low-pass filters 7 for converting the analog baseband complex video signal to a digital signal, and multipliers 9 for weighting the digital signals output by the A/D converters 8 to adjust the sidelobe levels in the beamforming process. Receivers 10 comprise the preceding components 3 to 9. A digital multiple beamformer 11 performs mathematical operations on the outputs of the receivers 10 connected to the respective antenna elements 1 to create multiple beams corresponding in number to the number of antenna elements.
This radar operates as described next.
The radio-frequency signals received by the N antenna elements are amplified by the RF amplifiers 3, then down-converted by the mixers 4 to an intermediate frequency and amplified again by the IF amplifiers 5. The phase of the intermediate-frequency signals is detected by the phase detectors 6, which convert the signals to complex video signals comprising an I-channel component and a Q-channel component. The complex video signals are band-restricted by the low-pass filters 7, converted to digital complex video signals by the A/D converters 8, and weighted by the multipliers 9 to reduce the side lobes in the beamforming process, then supplied to the digital multiple beamformer 11. The direction in which the N antenna elements are aligned is shown as the x-axis in FIG. 2. Let .alpha. be the angle of the incoming RF wavefront with respect to the x-axis, let d be the antenna element spacing, and let .lambda. be the wavelength. The phase difference between the signals received by adjacent antenna elements is then 2.pi.(d cos .alpha.). The digital multiple beamformer 11 can simultaneously create N beams (r=-N/2, . . . 0, . . . , N/2-1) having a maximum gain at .alpha.r=cos.sup.-1 (r.lambda./Nd) by performing the following calculations: ##EQU1## where Wk is a weighting coefficient introduced by the multipliers in the receiver 10 to suppress side lobes.
The beam width .delta.r of the r-th beam is given by equation (2) below: EQU .delta.r=w.lambda./Nd sin .alpha.r (2)
The interval .DELTA..alpha.r between the r-th beam and the r-1-th beam is given by the equation (3): ##EQU2## In equation (2), w is a constant determined by the coefficients Wk, and is generally set in the range from about 0.88 to 1.3. In the digital multiple beamformer 11, equation (1) describes a discrete Fourier transform (DFT). Holographic radars therefore employ the fast Fourier transform (FFT) algorithm to efficiently form a multiple beam in the field of observation, which is the coverage field determined by the antenna element beam width.
Due to the configuration described above, the field of observation of prior-art holographic radars is limited by the antenna element beam width (approximately 120.degree.). To scan the entire 360.degree. field it is necessary for the radar to be rotated mechanically, which lengthens the time required for a 360.degree. scan, or for the field to be divided among three or four radars, which increases the cost of the apparatus.
Another problem is that a separate receiver is required for each antenna element, in contrast to a phased-array radar which performs analog beam forming in the RF stage. The large number of receivers adds significantly to the size and weight of the apparatus, and the receivers consume considerable power.